The subject of the invention is a method of forming ribs on pipes or tubes, especially ribs of small pitches or ribs on tubes of low-plastic materials.
Hitherto, production of ribs on tubes by the drawing method, called also the rolling method, has been carried out in heads comprising usually three symmetrically arranged cylinders composed of a series of roll-formed disks. The axis of each cylinder forms with the axis of rolling, which is the axis of the tube, a constant angle. The value of the said angle results from the geometrical dependencies of the helical outline of ribs drawn on the tube. Cylinders are driven with rotary motion, due to which, after introducing the tube into the zone of the action thereof, the cylinders transmitting the rotary motion to the tube draw the rib. Forming the ribs on the tube is the result of producing in the zone of the action of the tool a corresponding state of stresses in the material of the tube, causing a plastic strain. The material of the tube deforms in the zone of the action of the tool in the axial and radial direction. Radial strain manifests itself in a form of increasing the diameter of ribs, and axial strain manifests itself in a form of elongation of the tube. Elongation of the tube in the process of drawing the ribs is a disadvantageous phenomenon because the quantity of the elongation is random variable for each segment of the tube. This characteristic causes the necessity of applying technological allowances, and thus, production of wastes. Axial strains of the tube are caused by corresponding axial stresses induced by the tool in the zone of its operation. As a result of the action of the said stresses the tool is subject to very high loads in the axial direction. The said loads cause bending out the disks of which the tool consists. As a result thereof the pitch of ribs is not uniform and always greater than the pitch of the disks. The substantial problem in the hitherto applied methods of drawing the ribs is a low durability of the tool whose disks are destroyed in the result of the action of bending loads of a fatigue character.
The Polish patent specification No. 113419 presents the design of a tool with the use of an additional disk stiffening the tool and increasing its durability. Such a solution does not eliminate, however, the elongation of the tube and it is difficult to draw ribs of very small pitches on tubes of materials having high resistance to plastic strain because the increase of loads in the axial direction of the tool is considerably greater than the possibility of increasing the rigidity of the tool.
The U.S. Pat. No. 3,876,593 presents a method of producing bimetallic ribbed tubes with the application of blocking the possibility of elongation of the tube on which ribs are drawn. This blocking consists in clamping the end of the external tube on the internal tube, and then the drawing process starts from the second, free end of the tube. During drawing ribs by the said method different values of axial stresses are observed along the length of the tube. In the initial stage, when starting drawing the ribs from the free end of the tube, axial stresses in the non-deformed part of the tube grow systematically till reaching a definite state of equilibrium of stresses in the material of the external tube. At the same time, as a result of increasing axial stresses, in the external tube, from the value "0" at the beginning to the value corresponding to the state of equilibrium, the diameter of ribs increases from a certain initial quantity to the required quantity. The segment of the tube with different diameters of ribs usually does not meet the requirements imposed to dimensional tolerances and is a waste material. A disadvantageous phenomenon in the first stage of drawing is also the excessive bending load of the disks of the tool.